My grandfather's family were all lactose intolerant (missing the mutation). They lived in a mountain area where raising cows was one the main food resource, so

My grandfather's family were all lactose intolerant (missing the mutation). They lived in a mountain area where raising cows was one the main food resource, so dairy product were practically present in almost every meal. 2 generations later I am the only one missing the mutation in my generation, everyone else has it (dozens on cousins). It seems that a combination of 2 parents where 1 has the mutation and 1 is missing it results in most of the children inheriting the mutation. That explains the fast "conversion".